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Tuesday, July 19, 2011

Voltage Regulation of Transformer

1. Voltage Regulation of Transformer
Because of the voltage drop across the primary and secondary impedances it is observed that the secondary terminal voltage drops from its no load value (E2) to load value (V2) as load and load current increases.

This decrease in the secondary terminal voltage expressed as a fraction of the no load secondary terminal voltage is called regulation of a transformer.

The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load i.e. rated load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

Let E2 = Secondary terminal voltage on no load
V2 = Secondary terminal voltage on given load
then mathematically voltage regulation at given load can be expressed as,

The ratio (E2 - V2 / V2 ) is called per unit regulation.

The secondary terminal voltage does not depend only on the magnitude of the load current but also on the nature of the power factor of the load. If V2 is determined for full load and specified power factor condition the regulation is called full load regulation.

As load current increases, the voltage drops tend to increase V2 and drops more and more. In case of lagging power factor V2 < E2 and we get positive voltage regulation, while for leading power factor E2 < V2 and we get negative voltage regulation.

The voltage drop should be as small as possible hence less the regulation better is the performance of a transformer.

We have seen that for lagging power factor and unity power factor condition V2 < E2 and we get positive regulation. But as load becomes capacitive, V2 starts increasing as load increase. At a certain leading power factor we get E2 = V2 and the regulation becomes zero. If the load is increased further, E2 becomes less than V2 and we get negative regulation.